1. Field of the Invention
The present invention relates to a method for polishing semiconductor wafers, which are mounted on a front side of a support plate and one side face of which wafer is pressed, by means of a polishing head, against a polishing plate, which is covered with a polishing cloth. A specific polishing pressure is applied and the wafer is polished. The present invention also relates to a device which is suitable for carrying out the method.
2. The Prior Art
Making a semiconductor wafer planar by means of a chemical/mechanical polishing method forms an important processing step in the process sequence to produce a flat, defect-free and smooth semiconductor wafer. In many production sequences, this polishing step constitutes the last shaping step. Hence, this step decisively determines the surface properties of the wafer, prior to the further use of the semiconductor wafer as starting material for the production of electrical, electronic and microelectronic components. An objective of the polishing method includes, in particular, the achieving of a very high degree of evenness and parallelism of the two wafer sides. Other objectives include the removing of surface layers which have been damaged by pretreatments ("damage removal") and reducing the microroughness of the semiconductor wafer.
Single side and double side polishing methods are usually employed. The present invention relates to the single side polishing of a batch of a plurality of semiconductor wafers ("single side batch polishing"). In this method, one side of each semiconductor wafer is mounted on the front side of a support plate. This mounting is by producing a positive and force-fitting connection between the wafer side and the support plate, for example by means of adhesion, bonding, cementing or the application of a vacuum. Generally, the semiconductor wafers are mounted on the support plate in such a way that they form a pattern of concentric rings. Following the mounting, the free wafer sides are pressed against a polishing plate, over which a polishing cloth is stretched. Then a supply of a polishing abrasive is provided at a specific polishing pressure and the free wafer sides are polished. In the process, the support plate and the polishing plate are usually rotated at different speeds. The polishing pressure required is transmitted to the rear side of the support plate by a pressure punch, which is referred to below as a polishing head. A multiplicity of the polishing machines used are designed such that they have a plurality of polishing heads at their disposal and accordingly are able to accommodate a plurality of support plates.
A number of factors make it difficult to achieve the desired evenness and parallelism of the semiconductor wafers, hereinafter called the desired wafer geometry. The wafer geometry is unsatisfactory particularly for polished semiconductor wafers whose sides are not parallel to one another but rather assume the shape of a wedge. For example, deviations from the desired wafer geometry are already caused by slight unevenness on the rear side of the support plate. This unevenness results in an increased or reduced polishing abrasion on the semiconductor wafer lying opposite the unevenness. Even a wedge shape of a semiconductor wafer caused by the polishing is ultimately the result of a polishing pressure acting inhomogeneously on the semiconductor wafer and of a material abrasion which as a result is necessarily uneven. Therefore, the polishing pressure frequently does not act uniformly on the semiconductor wafer. This is because the support plate is deformed radially during the polishing by its own weight or has a certain production-related, radial wedge shape. With polishing heads of identical design, it is possible for there to be differences in the transmission of the polishing pressure. This has the effect that the polishing head used also makes its presence felt in the polishing result. On some occasions, incipient wear of the polishing cloth is also a cause of the wafer geometry deteriorating during the course of a plurality of polishing passes.
To ameliorate the above-mentioned problems during attempts to achieve the desired wafer geometry, it is proposed in EP-4033 A1 to insert intermediate layers comprising soft, elastic bodies between the polishing head and the rear side of the support plate. This method cannot be automated and is susceptible to errors. This is because its success is largely dependent on the experience and watchfulness of the operating staff, who have to select and insert the intermediate layers on the basis of their width. However, even if no errors are made in doing this, the wedge shape of the polished semiconductor wafers remains above a specific limit value.